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Merge #88

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88: crt0.s removed redundant lines, added comments r=Lokathor a=felixjones

The flags of `cpsr_cf` are left untouched, so `cpsr_c` is used instead.

Line 102 was redundant:
```asm
bic r2, r2, #0x1F
orr r2, r2, #0x1F
```
Now it explicitly switches from incoming IRQ mode to system mode.

The same type of explicit switch is done for system mode to IRQ mode (only clearing bits necessary), however the IRQ mode bits are or'd also just-in-case the user has modified `cpsr_c` in their IRQ handler.

The IRQ disable bit is also set, again, just in-case the user changed it.

Hopefully I've managed to communicate the behaviour in the ASM comments.

Co-authored-by: Felix Jones <felix@felixjones.co.uk>
This commit is contained in:
bors[bot] 2019-09-19 02:34:12 +00:00 committed by GitHub
parent 898fbdadd0 09a7c73004
commit 35e9857dcc
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GPG key ID: 4AEE18F83AFDEB23
1 changed files with 12 additions and 11 deletions
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crt0.s
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@ -51,12 +51,12 @@ __start:
@ set IRQ stack pointer @ set IRQ stack pointer
mov r0, #0x12 mov r0, #0x12
msr CPSR_cf, r0 msr CPSR_c, r0
ldr sp, =0x3007fa0 ldr sp, =0x3007fa0
@ set user stack pointer @ set user stack pointer
mov r0, #0x1f mov r0, #0x1f
msr CPSR_cf, r0 msr CPSR_c, r0
ldr sp, =0x3007f00 ldr sp, =0x3007f00
@ copy .data section to IWRAM @ copy .data section to IWRAM
@ -89,19 +89,19 @@ MainIrqHandler:
stmdb sp!, {r0-r2,lr} stmdb sp!, {r0-r2,lr}
@ Disable all interrupts by writing to IME @ Disable all interrupts by writing to IME
mov r0, #0 @ r2 (0x4000200) can be used as we only care about bit 0 being unset
strh r0, [r2, #8] strh r2, [r2, #8]
@ Acknowledge all received interrupts that were enabled in IE @ Acknowledge all received interrupts that were enabled in IE
ldr r3, [r2, #0] ldr r3, [r2, #0]
and r0, r3, r3, lsr #16 and r0, r3, r3, lsr #16
strh r0, [r2, #2] strh r0, [r2, #2]
@ Switch to system mode @ Switch from IRQ mode to system mode
@ cpsr_c = 0b000_10010u8 | 0b000_01101u8
mrs r2, cpsr mrs r2, cpsr
bic r2, r2, #0x1F orr r2, r2, #0xD
orr r2, r2, #0x1F msr cpsr_c, r2
msr cpsr_cf, r2
@ Jump to user specified IRQ handler @ Jump to user specified IRQ handler
ldr r2, =__IRQ_HANDLER ldr r2, =__IRQ_HANDLER
@ -112,11 +112,12 @@ MainIrqHandler:
.Lreturn: .Lreturn:
ldmia sp!, {lr} ldmia sp!, {lr}
@ Switch to IRQ mode @ Switch from ??? mode to IRQ mode, disable IRQ
@ cpsr_c = ( !0b000_01101u8 & cpsr_c ) | 0b100_10010u8
mrs r2, cpsr mrs r2, cpsr
bic r2, r2, #0x1F bic r2, r2, #0xD
orr r2, r2, #0x92 orr r2, r2, #0x92
msr cpsr_cf, r2 msr cpsr_c, r2
@ Restore IRQ stack pointer and IME @ Restore IRQ stack pointer and IME
ldmia sp!, {r0-r2,lr} ldmia sp!, {r0-r2,lr}